There are multiple mentions of von Neumann probes in The Future of Humanity by Michio Kaku. This hypothetical probe could be the way that humans branch out into the stars. These self-replicating spacecrafts would go to a distant planet and use any raw materials present to generate copies of itself which could then be sent out to even more distant planets. It’s easy to see how one probe that is capable of making one thousand copies of itself could each send a copy into space to make one thousand more copies, again and again over large time scales which would allow humanity to have an outpost or, at the very least, radio contact across the galaxy and eventually the universe.